Accelerometers and gyroscopes are in wide use today for a variety of motion sensing applications ranging from inertial navigation to vibration monitoring. Accelerometers measure changes in acceleration (linear or rotational) while gyroscopes provide information about angular motion (rotation). These devices use the inertial properties of light or matter for their operation and have hence been broadly classified as ‘inertial sensors’.
Inertial sensors have traditionally served navigation markets. Macroscale gyroscopes and accelerometers are used to provide information about the position, orientation and velocity for aircraft and naval vessels. They have also been incorporated into control systems that are used for robotic applications, such as missile guidance, unmanned aircraft, and industrial machine control. Commercial inertial navigation grade sensors are available from Honeywell Corporation, Northrop Gumman and Naysys Corporation, amongst others.
While these macroscopic accelerometers and gyroscopes still remain the premier instruments for inertial grade navigation systems, microscopic mechanical inertial sensors fabricated using MEMS technology have been perceived as a breakthrough in inertial navigation and motion sensing, due to the substantial reduction in cost, size and power that may be achieved in such micromechanical sensors relative to their macroscopic counterparts. These attributes have enabled the use of such inertial sensors in a variety of applications that have traditionally not been possible due to either their prohibitive cost or size restrictions, for example, in mobile phones, MP3 players, PDAs, notebooks, surgical instruments etc. Commercial MEMS inertial sensors are now available from Analog Devices Inc., Motorola, ST Microelectronics and Silicon sensing systems amongst several others.
Micro-machined MEMS resonant accelerometers and gyroscopes are well known in prior art. See for example, U.S. Pat. No. 7,640,803, CN101303365 and U.S. Pat. No. 5,969,249. Most of the disclosed MEMS resonant inertial sensors respond to acceleration or angular rotation by producing a frequency shifted output signal arising from an oscillating unit incorporated as a part of the sensor.
Thiruruvenkatanathan et al, in a paper entitled “Ultrasensitive mode-localized micromechanical electrometer”, Frequency control symposium (FCS), 2010 IEEEInternational, IEEE, Piscataway, N.J., USA, 1 Jun. 2010, describes a MEMS resonant sensor based on mode localization. The system described relies on weakly coupled resonant elements in which one resonant element is fixed to a suspended mass. Movement of the suspended mass introduces a differential axial strain on the resonant element to which it is mechanically fixed, leading to a change in the modal response of the weakly coupled resonant elements. This mode shape variation was proposed as a new sensing mechanism for the realization of inertial sensors that were based on measuring induced strain as a function of an input inertial force.
However, a measure of strain in such inertial sensor implementations necessitates the use of compliant resonator topologies that require operation in a low pressure (vacuum) environment in order to provide for sufficiently high quality factors and hence sufficiently high resolution for most commercial applications. This consequently imposes stringent limitations on the packaging of these sensors and consequently, increases the cost of manufacture. It is an object of this invention to overcome these disadvantages by allowing for the realization of a mode-localized MEMS resonant sensor with sufficient resolution that is operable at atmospheric pressures.